1. Field of the Invention
This invention relates to a method for forming a pattern on a surface of a metal substrate, more particularly to a method for forming an optical interfering pattern on a surface of a metal substrate. This invention also relates to an article having a pattern of micro-cavities, more particularly to an article having a pattern of micro-cavities that exhibits an optical interfering effect on reflection of the pattern of micro-cavities.
2. Description of the Related Art
Up to now, a light-reflecting pattern with an optical interfering effect is formed by forming a plurality of cavities on a surface of a metal substrate using a cutting tool, such as a diamond cutter. The cavities formed on the surface of the metal substrate are subsequently subjected to electroplating or anodic treatment so as to improve the light-reflecting effect or durability. However, when the width of the cavities is required to be in a level of hundreds of micrometers (μm), as shown in FIGS. 1 and 2, the conventional method for forming cavities on the metal substrates by using the cutting tool is merely capable of forming a pattern of parallel straight lines or concentric circles of cavities that have a V-shaped cross-section 9. A more complicated pattern compared to those shown in FIGS. 1 and 2 cannot be achieved using the conventional method. Therefore, when the conventional method using the cutting tool is applied to form a pattern having an optical interfering effect, the application and variation of the pattern thus formed is limited, and the optical interfering effect on the light reflection of the pattern occurs only within a small angle by virtue of the V-shaped cross-section 9 of each cavity.
As for the optical interfering pattern useful for anti-counterfeiting, such as rainbow halograms, which is widely used in different applications, such as credit cards, it is formed by printing and transferring techniques. Generally, such pattern is formed by the steps of: imaging the pattern having the desired optical information on a photographic film so as to form a rainbow halogram film, applying a photoresist material on the rainbow halogram film, patterning the photoresist material through exposure and developing using a laser, electroplating a nickel layer on the photoresist material and then removing the photoresist material so as to form a pressing mold, impressing the pressing mold on a transparent plastic film so as to pattern the transparent plastic film, coating the patterned transparent plastic film with an aluminum layer, and attaching the aluminum layer together with the patterned transparent plastic film to a plastic substrate. The desired optical information included in the pattern can be expressed when light is transmitted into the transparent film and is reflected by the aluminum layer.
In the abovementioned method for forming an optical interfering pattern using a rainbow halogram film, the steps involved are complicated. Moreover, the electroplating is limited to certain materials, the manufacturing cost is high, and the productivity is low. Besides, the optical interfering pattern is not directly formed on the substrate, and is instead indirectly attached to the substrate through an adhesive, which adversely affects the appearance of the resultant product, and which reduces the service life and application of the resultant product.